We have recently identified a new unfolded protein response (UPR) in the mitochondrial inter-membrane space (IMS-UPR). The focus of this application is on the role of the IMS-UPR in familial amyotrophic lateral sclerosis (fALS) associated with misfolded mutant SOD1. Our data indicates that accumulation of misfolded proteins in the IMS leads to the activation of AKT, which then promotes the phosphorylation and activation of the estrogen receptor alpha (ERalpha). In turn, activated ERalpha promotes the transcription of an array of genes aimed at reducing IMS-stress. We and others have demonstrated that mutant SOD1 localizes both to the cytoplasm and the IMS. Cell lines and mouse models were generated, in which SOD1-G93A is either in both the cytoplasm and the IMS or in the IMS-only. We used these models to test the activation of the IMS-UPR. Our preliminary data in cell lines and mouse models of SOD1-G93A ALS suggests that, while the IMS-fraction of SOD1G93A activates a cyto-protective IMS-UPR, the inhibition of the proteasome by cytosolic SOD1G93A fails to limit the import of the mutant protein in the IMS. As a result, sustained and unresolved IMS-stress occurs. We hypothesize that, under these conditions, mitochondrial damage increases and the IMS-UPR switches from being cyto-protective to being pro-death. We further hypothesize that this switch does eventually happen in the IMS-only model but at a much later time. Further, while the activation of the ERalpha by AKT is independent of estrogen, it nevertheless synergizes with estrogen. Since estrogen is known to be neuro-protective, the role of estrogen in the setting of the IMS-UPR remains to be defined. To test our hypothesis we propose the following specific aims: Specific aim 1: Monitoring the IMS-UPR throughout the natural history of the disease in ALS in SOD1-G93A and IMS-only SOD1-G93A transgenic mice. Specific aim 2: Testing the effects of ERalpha ablation on the natural history of the disease in the untargeted and IMS-only SOD1-G93A models. Specific aim 3: Dissecting the role of the ligand-dependent and ligand-independent functions of the ERalpha in the activation of the IMS-UPR.